1. Field of the Invention
The invention relates to a binding tape for adhesively binding pages together to form a `Perfect Bound` book or pamphlet, and more particularly, to an improved moisture proof thermally actuated binding tape.
It is often desirable to bind pages together permanently to form a `Perfect Bound` book, pamphlet, or the like. While numerous arrangements for doing this are known and available, one well known and popular method is to use an adhesive bearing binding strip or tape which when attached to the edge or spine of the assembled pages, binds the pages and the tape together to form the book. Binding tapes of this type typically employ either heat activated adhesive or a pressure sensitive adhesive, or combination thereof. The adhesive formulations used are many and varied.
Binding tapes employing heat activated adhesives generally use a heavy paper stock for the tape substrate or backing strip. However, paper has an inherent disadvantage in that paper is largely made up of cellulose fibers which are hygroscopic. When heated to activate the adhesive(s), any water in the paper is also heated, and since the binding temperatures typically used are normally above the boiling point of water, any water in the backing strip will be turned into steam.
Water is undesirable from at least two standpoints. First, the water imposes an additional thermal load during the binding process, especially during the period when the water changes from liquid to gas. Secondly, the steam produced from the water has a deleterious effect on the adhesive used. In the case of the latter, when the water reaches the boiling point and is converted into steam, the steam that results will try to exit the binding strip. Since escape from the non-adhesive bearing side of the backing strip is normally blocked or seriously impeded by the binding apparatus itself, the steam must escape at the interface between the strip and the adhesive. If the viscosity of the adhesive is low enough, the steam will bubble through the adhesive and escape to the atmosphere via this route. This bubbling however causes the viscosity of the adhesive to be mechanically lowered, changing the adhesive's properties and binding performance.
If the viscosity of the adhesive is high, release of the steam from the binding strip is inhibited until the adhesive reaches a viscosity where the steam can penetrate it. The steam is usually in the form of small bubbles of a size that is probably determined by the fiber structure of the paper. But, as the steam enters the adhesive, it collects and forms larger and larger bubbles. These larger bubbles are forced upwards by the pressure below and the effect of gravity, and as the bubbles rise, they lift the layer of adhesive above them. This changes the heat transfer relation from the more efficient conduction through the adhesive to the less efficient convection through the steam bubbles. This causes premature cooling and thickening of the adhesive, trapping the steam permanently inside the matrix of the book.
Where binding takes place in moderate humidity conditions, there typically results undesirable and unsightly `bumps` in the spine of the book. On the other hand, where binding takes place in high humidity conditions, the binding may not take, resulting in separation of the backing strip from the spine of the book, either immediately after binding or at some later point in time.
2. Description of the Related Art
In the prior art, U.S. Pat. No. 3,847,718 to Watson, which is incorporated by reference herein, discloses a binding means employing a relatively thick central stripe of low tack heat activated adhesive on a substrate such as paper flanked by relatively thin side stripes of high tack heat activated adhesive. A related patent, U.S. Pat. No. 4,244,069 to Hale, discloses a binding material in which the substrate comprises a heat shrinkable plastic covered with a pressure sensitive adhesive bearing a relatively thick heat activated adhesive stripe along the center thereof. U.S. Pat. No. 3,825,963 to Abildgaard et al discloses a combined mechanical/adhesive binding apparatus in which rivets or studs are used along with a binding tape to make books, the binding tape using a pressure sensitive adhesive to attach the tape to the spine of the book. And, U.S. Pat. No. 4,686,133 to Nakabayashi et al, discloses a material for sealing the mouths of glass containers in which a metal foil with a thermal adhesive layer bonded thereto by a polyester-isocyanate type two component adhesive is employed.